Hi there,The 240 volt can get a bit hot and the 12 volt ones seem to run much cooler IME
Its the needle valves that are more susceptible the flow rates changes with temps - esp the cheapo needle valves most of us useIf we get hold of the plastic sides of our Mains solenoid you can hold it all the time, just warm, but ours has a metal end to it and thats much hotter but not burning.
One thing to note, the heat from the solendoid may affect the bubble counter, on ours, if you adjust it when just switched on, Cold, you may find after a hour or so the bubble rate changes as heat ( slight) builds up in the whole unit, so best to readjust again.